This invention relates to foam materials and particularly to a method for manufacturing shaped articles comprising a foam having an integral non-foamed skin layer on at least a part of the surface of the foam.
Shaped articles, for example sheet materials, are known which consist of a foamed polymer core having a dense covering formed integrally with the core. In these known articles the density of the material of which they are made decrease steadily from the outer surface towards the centre. The articles are made by foaming a foamable mixture of the precursors of cured resins in which a blowing agent is incorporated, at a temperature sufficient to cause foaming, in closed moulds which are maintained at a temperature below that at which foaming can occur. In this way a temperature difference, usually of 20.degree. C. or more, is established between the centre of the mixture and its external surface which is in contact with the mould, with the result that the mixture in the centre is foamed whilst the mixture in contact with the mould is substantially prevented from foaming. The mixture may comprise the precursors of a polyurethane or the precursors of a cured polyester resin, that is an ethylenically unsaturated polyester and an ethylenically unsaturated monomer which is copolymerisable with the polyester. After foaming is completed the polymer is cured, either by a polyaddition reaction in the case of a polyurethane or by a copolymerisation reaction in the case of a cured polyester resin. Alternatively the mixture may comprise the precursors of a cured polyester resin and a polyisocyanate, as described in British Pat. Specification No. 1,308,637, in which case curing will be by both polyaddition and copolymerisation.
This known method of producing shaped articles requires strict temperature control in order to ensure a dense covering in which minimum foaming has occurred whilst ensuring that the core is foamed satisfactorily, and is difficult to operate with satisfactory and readily reproducible results. Moreover the resulting articles have a steadily decreasing density from the outer surface towards the centre and the process is unsuitable for the production of articles having a non-foamed skin layer and a foam body of uniform density. The process is unsuitable for the production of articles having clearly defined skin and foam layers and is unable to produce articles having a thin skin layer. The articles produced by the process have a very low volume:weight ratio compared with articles made by completely foaming the mixtures and is wasteful of starting materials. Further the process produces an article wherein the foam core is completely covered by a skin and is not readily adaptable to the production of articles wherein only a portion of the surface of the foam, for example one side of a sheet of the foam, is covered by a skin.
We have now devised a method for the production of shaped articles comprising a polymeric foam of uniform density having one or more substantially non-foamed skin layers over all or part of its surface, which method is easily carried out and does not require establishment of a temperature difference between the external surface of the foamable mixture and its centre and which obviates the need for strict temperature control of the foaming reaction. In our method, which employs a foamable mixture of a polymerisable material, the skin layer or layers is/are formed before the foaming reaction is commenced and thereafter the remaining portion of the mixture is foamed and the foam subsequently set by polymerisation of the polymerisable material. The products of our process have clearly defined skin and foam layers, and very thin skin layers, for example layers of thickness 10 thousandths of an inch or less, can be produced. The foam body is substantially completely foamed material so that the products have a high volume:weight ratio compared with the products of the known process.